Owing to the penetration of an Internet at ordinary homes and the progress of digitization of image media, the importance of high-speed communication infrastructure in a gigabit class is growing. An optical communication system is expected as such a high-speed communication infrastructure.
By the way, one of obstructions of introducing such a high-speed system into a subscriber loop such as ordinary homes is the compatibility of performance of the system and cost.
Generally, an optical communication system is expensive although being superior in communication speed and communication quality in comparison with a radio system, which becomes the hindrance of spread.
In particular, in the case of the construction at an ordinary home, an optical module for optical-electric conversion is required for the home, and it is indispensable to supply this at a low cost. Thus, the establishment of technique for supplying an optical module, having the rapidity that is a feature of the optical communication, at a low cost is very important.
A PLC platform (a mount substrate equipped with an optical circuit, PLC: Planar Lightwave Circuit) is proposed as what is used for an optical module that realizes high-speed optical-electric conversion exceeding gigabit (refer to page 59 of “The second collection of latest data on optical communication technique” published by Optronics Co., Ltd.). In addition, the entire disclosure of page 59 of reference, “The second collection of latest data on optical communication technique” is incorporated herein by reference in its entirety.
FIG. 8 is a drawing showing a basic structure of an optical mount substrate that forms the PLC platform by conventional technique.
A PLC section 810 comprises a part of a substrate 80 (right portion of the substrate 80 in FIG. 8), and a quartz-based optical waveguide (an upper cladding 81, a core 82, and a lower cladding 83) formed in an upper part thereof, and performs optical signal branch and synthesis.
The optical device 84 such as a laser and a photodiode is mounted in an optical device mounting section 820 which is another part of the substrate 80 (a central part of the substrate 80 in FIG. 8), and performs optical-electric and electric to optical signal conversions. An electric wiring section 830 which is the remaining portion (a left portion of the substrate 80 of FIG. 8) of the substrate 80 connects a drive circuit with the optical device 84, and transmits a high frequency that is high-speed equal to or more than one GHz.
Reasons why silicon is used as a material of the substrate 80 are as follows.
Namely, the followings can be cited:
(1) Silicon is suitable for the formation of an optical waveguide which needs an elevated temperature process,
(2) Since silicon has good workability, a V groove for optical fiber alignment is easily processible, and
(3) Since having good thermal conductivity, silicon acts as a heat sink even if a laser or a semiconductor IC that acts as the optical device 84 is driven in high power, and hence it is possible to suppress the rise of device temperature.
Since silicon is excellent in heat radiation effectiveness in this manner, the optical device 84 is directly mounted in the substrate 80 made of silicon.
On the other hand, since silicon has comparatively large dielectric loss, a parasitic inductance and a parasitic capacitance in the electric wiring section 830 become problems on the occasion of using a high-frequency band in the above-described optical mount substrate.
Then, in order to reduce the dielectric loss in high frequencies as much as possible, as shown in FIG. 8, electrodes are formed in the electric wiring section 830 with the electric wiring 85 such as a coplanar line, and quartz-based glass 86 with small high-frequency loss is made into a thick film, and intervenes between the electric wiring 85 and substrate 80.
In addition, in order to perform the height alignment of the optical device 84 and PLC portion, a silicon terrace 87 that has a terrace-like cross section that exists only in this portion is provided.
Thereby, the transmission and reception of a digital signal in a several gigabit class become possible.
However, on such a PLC platform, a production process is very complicated and results at a high cost.
That is, the photolithography and etching for forming the silicon terrace 87 in a silicon substrate 80 are required. Furthermore, it is necessary to repeat multiple times steps such as photolithography, etching, thin film deposition, and high-precision polishing for forming the upper cladding 81, core 82, and lower cladding 83. In addition, the upper cladding 81, core 82, and lower cladding 83 constitute an optical waveguide.
The cost weight of the optical waveguide is especially high. Although the optical waveguide is manufactured with using semiconductor processes such as film formation by a flame hydrolysis deposition or a CVD, and core patterning by photolithography and etching, cost reduction effectiveness is not expectable even in mass production since chip size is comparatively large.
Another problem is that it is not easy to connect the optical waveguide with an optical fiber.
In order to suppress the optical loss due to the connection between the optical fiber and optical waveguide, in the case of a single mode, positioning, assembly, and fixing of ±1 μm or less are needed.
As the connection methods, the following two are common.
One is to form a V groove for arranging the optical fiber into a silicon portion of an edge (not shown in FIG. 8) of the optical waveguide, when the optical waveguide (the upper cladding 81, core 82, and lower cladding 83) is formed in the silicon substrate 80, as shown in FIG. 8. The optical fiber is connected with the optical waveguide by arranging and fixing the optical fiber to this V groove.
However, there are problems that not only a cost burden increases since another steps such as photolithography and wet etching are required for forming the V groove in the silicon substrate 80, but also the dispersion of the optical loss amount in the connection of the optical waveguide and an optical fiber arises in consequence since etching dispersion arises and hence the geometry accuracy of the V groove disperses.
Another is a method of independently and individually preparing a substrate that forms the optical waveguide, and a substrate in which the V groove, where the optical fibers are arranged, is formed, and performing the alignment of these optical axes by a system equipped with several axes of automatic adjusting mechanisms. Nevertheless, there are many and large problems in respect of mass production property and economical efficiency since it needs the time from dozens of seconds to several minutes to adjust every connection and facility is also expensive.
As methods of solving problems relating to the manufacture of such an optical waveguide and the connection of the optical waveguide and optical fiber, methods of manufacturing an optical waveguide by forming a groove for fixing and a groove corresponding to a core of the optical waveguide in a fiber by press-forming, which has been already put in practical use as a manufacturing method of aspheric surface glass, and filling a core material such as a resin in the groove corresponding to the core are proposed in Japanese Patent Laid-Open No. 7-287141, Japanese Patent Laid-Open No. 7-113924, Japanese Patent Laid-Open No. 7-218739, etc.
In addition, the entire disclosure of reference “Japanese Patent Laid-Open No. 7-287141”, “Japanese Patent Laid-Open No. 7-113924”, and “Japanese Patent Laid-Open No. 7-218739” are incorporated herein by reference in its entirety.
This method is, for example, to transfer inversion geometry by pressing a mold die, which is equipped with concave and convex geometry corresponding to a fiber fixing portion and an optical waveguide portion, to a processed object as shown in FIG. 9, and can manufacture plenty of fiber fixing guides and optical waveguide grooves with sufficient geometry reproducibility. Here, “91” denotes an optical fiber guide groove forming section, and “92”, denotes an optical waveguide pattern molded section.
If core material such as a resin is embedded in the optical waveguide groove, it is possible to make the optical waveguide groove function as an optical waveguide, and hence, it becomes possible to optically connect the optical fiber with the optical waveguide simply and highly efficiently without performing special positioning just by arranging the optical fiber into the groove by transferring geometry with a die where relative positions of respective grooves for the fiber and optical waveguide are accurate.
Thus, owing to the use of the press forming, as shown in FIG. 10, it becomes possible to realize an optical mount substrate 103 having an optical waveguide 101 and an optical fiber guide groove 102, and hence it becomes possible to reduce the manufacturing cost of the optical waveguide and the connection cost of the optical fiber.
However, if optical devices such as a laser and a photodiode, and circuits which drive these are provided in such an optical mount substrate 103, there is a restriction that heat cannot be easily released and hence the optical devices can be driven only in low power since it is necessary to form them on glass having thermal conductivity worse than silicon.
In addition, in any one of the optical mount substrate 103 in FIG. 10 and the PLC platform in FIG. 8, since a leading lead is needed if an electric device such as a capacitor is provided in an electric wiring (“85” in FIG. 8 and not shown in FIG. 10) used as an electrode portion, loss arises in a high-frequency band, which becomes an abuse to acceleration.
Thus, in the present situation, there is a subject that optical mount substrates and optical modules that have the rapidity in a gigabit class, cost which is suitable for being supplied to ordinary homes, and productivity cannot be supplied.